Antennas of the frequency-reusing kind are known to operate simultaneously and independently with two orthogonal polarizations. They comprise a system of reflectors, of the Cassegrain type for example, a primary source of radiation, and a periscope ensuring the transmission of the radiated beam from the source to the reflectors; the periscope is formed by an assembly of four mirrors whose curvatives determine the operating frequency.
The system of reflectors is movable along two mutually perpendicular axes, namely elevational and azimuth axes respectively designated EL and AZ in FIG. 1 which shows a conventional single-band antenna.
In FIG. 1, the supply device comprises a fixed primary source 1 of the corrugated-horn type centered on the AZ axis. The periscope comprises a cylindrical structure 2, movable around this axis AZ, and four mirrors 3, 4, 5 and 6, the mirrors 3 and 6 (or possibly 4 and 6) being flat while the other two mirrors have a focusing (paraboloidal or ellipsoidal) curvature. An outgoing microwave beam is reflected successively by these mirrors 3, 4, 5 and 6.
The disposition of these mirrors in the periscope is well known for frequency-reusing antennas. Thus, the first mirror 3 is centered on the azimuth axis AZ; mirrors 3, 4 and 5 are integral with the structure 2 and therefore movable around the azimuth axis, whereas the fourth mirror 6 is integral with a rotatable and pivotable assembly including a main reflector 7 and a secondary reflector 8, being therefore movable around the azimuth axis as well as around the axis of elevation EL. This fourth mirror 6 is centered on the intersection of the azimuth and elevation axes.
In the field of spatial telecommunications it is desirable to utilize the same frequency-reusing antenna for two frequency bands, i.e. to associate a second band of higher frequencies with the single band of lower operating frequencies employed with the antenna described above. In a specific instance, the lower band ranges from 4 to 6 GHz and the higher band extends from 11 to 14 GHz. Therefore, a certain minimum alteration of the single-band antenna described above is needed to permit dual-frequency operation.
To this end it is necessary in all cases to attach a second supply device to the antenna to emit the waves of the higher band. It is impossible to place two horns on the azimuth axis of the antenna, as is done with the horn 1 in FIG. 1, as one inevitably would shield the other. An alternative positioning of the two supply devices and the periscope must therefore be adopted.
According to a prior-art solution shown diagrammatically in FIG. 2, a periscope 60 differs from structure 2 of FIG. 1 in that the first, flat mirror 3 is replaced by a dichroic mirror 30. This dichroic mirror is a device which is transparent for one band and reflective for another and which thus permits the combination of two separately transmitted beams into one. Inversely, it also permits the separation of received beams of different frequencies. There are at least three well known structures for this type of mirror:
a rectangular-mesh parallel-grid structure of metal wires or strips impressed on a very thin support of the mylar type,
a structure of parallel metal grids pierced with cross-shaped slots,
a network of waveguides cutting off one frequency band and passing the other.
A particular mirror structure of the first of the three types mentioned above is described in an article entitled "Quasi-Optical Polarization-Independent Diplexer" which appeared in the IEEE review of November 1976, pages 780 to 785.
The system of reflectors 7 and 8 in FIG. 2 remains unchanged from FIG. 1 because the example of the Cassegrain antenna considered above is retained.
There are two supply devices, namely a lower-band feed 40 and a higher-band feed 50. Feed 40 comprises a horn 1 identical to that of the single-band antenna, the dichroic mirror 30 playing the same part as the flat mirror 3 of FIG. 1. The dichroic mirror 30 is a "high pass" mirror whose cut-off frequency is such that it reflects the waves transmitted by the horn 1. These waves therefore follow a route identical to that of the waves in the single-band antenna.
The supply device or feed 50 of the higher band comprises a primary source 10 and a concave focusing mirror 9. The primary source 10, for example a corrugated horn, is parallel to the azimuth axis and like horn 1 radiates in an upward direction. The mirror 9 is situated so that the radiation beam of the higher band, which it reflects, is aimed at the dichroic mirror 30 and is superimposed on the reflected beam of the lower band beyond this mirror.
By suitably arranging the relative distances of the source 10, the focusing mirror 9 and the dichroic mirror 30, as well as the curvature of the mirror 9, there is obtained in the region of the dichroic mirror 30 a focusing of the field of the higher band similar to that undergone by the lower band.
In the assembly of FIG. 2, purity of polarization is obtained, as in the single-band antenna of FIG. 1, by a mutual compensation of the crossed polarizations created by each of the two focusing mirrors 4 and 5. It is sufficient to shape the central areas of these mirrors with great accuracy in order to obtain the same effect for the higher band as for the lower band.
It is seen in FIG. 2 that when the periscope 60 rotates around the azimuth axis AZ, it is necessary for the higher-band supply device 50 to carry out the same rotation; this device 50 is therefore mechanically linked to the periscope 60 which entrains the primary source 10.
One of the major drawbacks of this prior-art system is the mobility of source 10; since that source must in turn be supplied by a waveguide, not shown, the construction of the latter is difficult to realize.